This invention relates to the field of petroleum production from oil bearing geological formations, and more particularly to various methods of selective electrical resistance heating for facilitating the recovery of oil from locations that are not normally susceptible to commercial recovery by fluids injected for secondary or tertiary recovery purposes.
The progressive depletion of domestic oil reserves has generated substantial development work directed to methods for secondary or tertiary recovery. A common secondary recovery method that has received substantial commercial use is flooding by means of a fluid, such as water or steam. In such flooding methods the fluid is typically injected into a formation at an injection well for the purpose of driving oil from a porous zone of the formation toward a production well, where it is recovered. Although substantial amounts of oil can be recovered by flooding, it is not possible to recover all of the oil contained in the formation. There are a number of limitations which prevent exhaustive recovery of the oil from a formation by flooding techniques.
Petroleum which is not subject to primary recovery is typically distributed along with connate water in porous rock or sand. Throughout this specification, the terms "oil" and "petroleum" refer to crude oil, including high molecular weight hydrocarbons that are sometimes referred to in the art as "tars". If the oil in a reservoir is of relatively high viscosity, an injected fluid tends to channel through the oil zone of a porous geologic formation rather than displacing oil toward a production well. In many cases it is impractical to achieve adequate flow without heating the oil to reduce the viscosity. Thus, water or fluid flooding is sometimes carried out with hot water or steam. In numerous instances, however, even the use of steam flooding is not practically effective for heating the oil content of the reservoir and effecting its movement through the formation to a recovery well. Thus, for example, if the reservoir is at too great a depth, steam heating may not be economical. In certain other cases steam heating may be ineffective for recovery from a portion of the reservoir because of very low permeability, inaccessibility, or pressure limitation.
Many formations contain layered reservoirs in which the permeability of the layers differs and injected fluids preferentially flow through the more permeable layers, largely bypassing the less permeable layers. Once the more permeable layers are depleted as a result of fluid injections, further recovery is generally uneconomical because either the rate of fluid penetration into the low permeability zone is too low, or fluid bypassing through the more permeable zones causes the production of an excessive ratio of injected fluid to oil at the production well. Schemes for avoiding this effect include plugging of the more permeable zone and selective well completion, but such schemes are expensive and frequently ineffective.
In order to promote the recovery of oil by flooding, proposals have been made to utilize electrical resistance heating. As described, for example, in Crowson et al U.S. Pat. No. 3,605,888, resistance heating is utilized to provide hot water or steam in the hydrocarbon zone in the well for use as a flooding medium and to reduce the viscosity of oil in the reservoir. However, the commercial application of electrical resistance heating has been inhibited by the relatively high cost thereof. Thus, it is generally not competitive simply as a means for generating steam, and direct steam injection is less expensive than electrical resistance heating for reducing oil viscosity. Thus, as a general energy source for facilitating secondary recovery, electrical resistance heating has been less attractive than older and more conventional techniques.
Despite their usefulness and cost advantages over resistance heating for general secondary recovery purposes, the hot water flooding and steam flooding techniques conventionally used in the art have, as noted above, not been effective to recover all the potentially available oil, particularly that in relatively inaccessible locations such as deep reservoirs, low permeability formations and the normally bypassed regions of a pattern flood.
The secondary recovery of low or moderate viscosity oil is frequently accomplished by the injection of unheated water. This technique is effective for recovering oil from portions of the reservoir that are swept by the injected water, but water flooding frequently bypasses oil in low permeability zones and in unswept portions of the flood pattern. Thus a technique is needed for recovering oil that is bypassed by a water flood or other recovery technique. More generally, a need has remained for improved methods which are capable of reducing oil flow resistance, and thereby increasing the recovery of oil from otherwise inaccessible regions.